Grooved piston



FIG. 1 is a perspective view of a top perspective view showing the new design for a grooved piston;

FIG. 2 is a side elevational view thereof;

FIG. 3 is a top plan view thereof; and,

FIG. 4 is a bottom plan view thereof.

The portions of the grooved piston shown in broken lines form no part of the claimed design. 

The ornamental design for a grooved piston, as shown and described. 